TESTING WITH ULTRA VIOLET (UV) LIGHT
How UV works, blacklight, wavelengths explained, several UV torches reviewed
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Ultra violet light (known is the U.S.A. as 'black light') is a very intense blue light, it is not visible to the human eye. Fluorescent-tube UV torches give off a very dull purple or violet glow. LED UV torches give off a dull lilac-blue. But the actual UV light is invisible, so the only way you can tell how effective it is - is to observe its effects.
If porcelain has been repaired, the missing part will have been 'rebuilt' or the crack glued, and then carefully repainted so that the colour matches. The colour will match perfectly in white light (e.g. daylight) but under UV light the new paint shows up a totally different shade. This applies to many decorative items, watch dials, dolls heads, faint signatures on letters, modern touch-up paint on old paintings. You have probably noticed a similar effect when looking at a car parked under a yellow street light, new paint on the car can show up quite a different shade, whereas in daylight it matches perfectly.
Other examples in the world of antiques and art: oil paintings, art on paper, marble, jade and ivory, all show characteristic patterns under UV light:
How does it work? It works by reflecting off the surface and showing up the difference in the chemical composition of the paint, or by showing characteristics of the item's basic chemical make-up.
Examples from every day life.
Many everyday substances glow under UV light: chlorophyll, teeth and antifreeze; olive oil, honey and ketchup; some cosmetics, some drugs, some postage stamps and some flowers.
Any object painted with UV paint will glow under UV light, common uses are finding gold balls, tortoises, even your keys...providing you will be searching for them in the dark.
There is starch in ordinary paper that makes it glow a vivid blue when lit with ultra violet light, but paper banknotes (which are made of a linen-type material, not paper) merely look 'slightly blue', hence UV lights are used as 'forged banknote detectors'; some paper banknotes (and all plastic banknotes) are also printed with 'secret' marks, using UV-ink, which can only be seen under UV light - see some examples at the bottom of this page.
Additives in soap powders glow under ultra violet light, they are added for that very reason: UV light in daylight makes your white washing glow blue-white, so that the advertisers can say that their powder 'washes whiter than white'.
Quinine is an ingredient of some drinks (e.g. tonic water), it gives them a distinctive bitter taste. Quinine glows under UV light. So the next time you are in a dimly-lit bar drinking a gin and tonic, shine your UV torch on it and see it glow.
Cross linkers and light boxes for viewing gels in biological laboratories; germicidal UV lights for treating food before it goes on sale in the supermarkets; welding operations; for curing uv-sensitive adhesives or uv-sensitive nail varnish; for inserting into pipes carrying water to fish tanks or fish ponds to kill algae and bacteria; for treating skin wounds (though there is a balance, here, between the benefit of stimulating healing and the risk of causing cancer).
Animals and plants
Humans need UV light (from sunlight) to produce vitamin D in the skin (nearly everyone in Northern Europe is vitamin-D deficient in the winter due to lack of sunshine!); in the summer it's popular to take advantage of prolonged UV light from the sun to burn the skin (it's called a suntan) - and if there's no sun there are tanning parlours in which you can be gently grilled under giant UV lights. Over-exposure to UV light can cause cancer but some exposure is essential, the solution is to go out in the sun (without sun cream) whenever possible but don't stay out long enough to get a tan.
In humans, the lens of the eye filters out UV light so that we cannot see it, but some people who have had eye operations (e.g. following a cataract operation) can see UV, it appears as a bluish or purplish glow. An example of this is in the paintings of water lilies by Monet following his cataract operation.
For most humans, the only way to see the effect of UV light is to use a UV torch. For instance, to detect lichen, shade it from daylight, shine a UV light on it, and see if it fluoresces white, bluish or orange - if so, it's probably lichen (try it out on Cladonia portentosa).
The same applies to freshly-spilt blood and urine, the human eye can't see it fluoresce in daylight, but you can see it glow under a UV light in the dark (used by the the forensic services at crime scenes).
Most mammals, including man, cannot see UV light, but many other animals can. Kestrels can detect the urine trails of prey; to reindeer living in the Arctic urine and lichens appear black against the ice and snow as they absorb UV light (urine is bad, it indicates nearby predators whilst lichens are good, reindeers eat them); recent research indicates that many fish, reptiles and birds can see UV light.
Here are four questions we get asked about UV light.
Question: so why do some UV lights appear really bright and others appear really dim?
Answer: many popular UV torches are 'popular' because they appear 'really bright' to the human eye, but the amount of actual UV light (which is not visible to the human eye) is very small, and its effect is very slight...to the point of being useless. By contrast, some UV torches appear dim to the human eye, but the amount of actual UV light is large and they work well. We have tested each of our products and each one in our store includes the test results.
Question: do I have to use a UV torch in the dark?
Answer: the effect is certainly spectacular if you go into a completely dark room, but usually it is sufficient to use it in dim light. Use a square of dark cloth, or the underneath of a table in a dim corner or even the inside of your jacket (you may have seen antiques dealers lifting their jacket and, it would seem, examining their armpit with a blue light). UV lights used in shops for testing banknotes include a shade. This is no different from using an ordinary torch, turn it on outside in bright sunlight and you won't see any effect.
Question: can't I just use a brighter UV torch, so that I don't have to use it in 'dim' light?
To an extent
- yes. But it depends on the type (wavelength) of the light, if it is
wrong (as with all the cheap UV torches) then it won't work, no matter
how bright the UV light. Providing the type (wavelength) is right, then
yes, a larger brighter UV torch will light up a wider area and work in
brighter light, just the same as an ordinary torch. Scroll down or click
here for recommendations.
Question: if I use it by shining the light on the surface of an antique, does that mean that it won't see below the surface?
Answer: Exactly so! If the item has been repaired and then re-glazed, a UV torch will be of no use, there will be no 'new' paint to see, the entire surface will be new. Similarly, you cannot see through layers of paint to see 'hidden paintings', UV light is not the same as x-rays. But you can see differences in surface paint, and that can indicate that a painting has been 'touched up'. Similarly, if a signature reacts differently to the surrounding work, it's clearly made of a different paint and may have been added.
The electromagnetic spectrum ranges from AM at one end (which includes Medium Wave and Long Wave radio) to Gamma Rays at the other. The measurements on the right of the chart, below, are the length of each wave section (from peak to trough) in nanometers (nm). 1nm = 0.000001mm.
As you see from the chart, "visible light" falls between microwaves and x-rays.
UV light, although close to x-rays, cannot penetrate the human body, its effect (for shortwave UV) is limited to burning the surface of skin (and eyes). Having your skin burnt by shortwave UV light is very popular, it's called 'getting a suntan', and if there is no sun, you can get yourself grilled under giant UV lights on a sun bed.
The tiny band we call 'light' ranges from infrared (the human eye can't detect far-infrared) to ultraviolet (the human eye can't detect ultraviolet):
And within this, the tiny bit we call 'ultraviolet light' looks like this:
365nm - 302mn - 254nm
UV light is divided into three types depending on the wavelength (these classifications are approximate, since there's nothing to distinguish the boundary from one to another):
UV, also known as UV-A, 400 to 300nm
All UV light is invisible to the human eye, the UV torches you buy also give out some visible light and it is this you can see as a dull glow. Manufacturers of the cheaper torches ensure that the 'dull' glow isn't too dull, so that the user says, "Wow, this UV torch is so powerful!". Giving out some visible light is, of course, useful, otherwise you wouldn't know if your UV torch was switched on or off.
All the UV torches we sell are UV-A / longwave, they are not shortwave (shortwave burns skin and can damage eyes, they are usually encased in safe 'viewing cabinets'). Here is a list of how the various types (wavelengths) of UV-A / long wave lights compare (my standard test is on British twenty pound note look for the "20" and also the myriad of fine lines by the portrait of the Queen)
410nm: technically this is UV, but it has no effect on a twenty pound note, it does not show up any of the security markings
395nm: the short lines are clearly visible but the "20" is not - not even if you use the it in the dark
the short lines are visible; the "20" is just about visible,
but only in the dark.
UVA, SUMMARY. All our UV torches are UVA (see above). It is quite safe to point these at the skin, there is no way this type of UV can burn skin. However, please do not look directly into the light.
UVA, DETAIL. UVA is not harmful to the skin but could (possibly) be harmful to the eyes if exposed for prolonged periods. A "prolonged period' is measured as 'exposure over an eight hour period'. What we don't know is, how long is a 'prolonged period'? For more than one hour in any eight hour period? For more than 10mns in any eight hour period? For more than 1mn? We really don't know, therefore: - to avoid any possibility of harming your eyes, don't look directly into the light.
UVB and UVC (but mostly UVC) can burn the eyes or skin. When used in sun beds they are specifically designed to burn (tan) the skin, but the user must wear dark glasses to protect the eyes. Prolonged exposure, such as over-sunbathing, can lead to skin cancer. We do not sell UVB or UVC torches.
There are many other uses of UVC which do not concern the antiques and jewellery trades. Examples are: cross linkers and light boxes for viewing gels in biological laboratories; germicidal UV lights for treating food before it goes on sale in the supermarkets; controlling algae and bacteria in water; welding operations; for curing adhesives and nail varnishes.
Susceptibility to skin being burnt by UV light depends on skin colour. Brown-skinned people are less sensitive to being burnt by UV light, melanin in the skin provides protection; completely black-skinned people have a variation, eumelanin, which gives even greater protection. This dates from the time when most humans lived in Africa. In populations that spread northwards, to lands where there wasn't much sunshine, fair skin evolved; it evolved to be more sensitive to UV light, because UV light is necessary for producing vitamin D in the skin.
to see examples of what you see under each of these lights - which gives
you an excellent idea of how they compare.
This works really
well but covers only a tiny area (a few millimeters). Good to carry in
your pocket, it's tiny, and includes a keyring.
One end is a UV
torch with 1 super-powerful UV L.E.D. The other end is an extraordinarily
powerful ordinary (white light) torch, which is useful for inspecting
antiques, e.g. shining white light through fine porcelain or into narrow
spaces. The UV light is not only of a good wavelength (365nm) but is also
finely-focused, the result is a UV light that is so powerful it even works
in daylight (though the effect is more spectacular in dim light). It's
powered by a built-in rechargeable battery, one charge gives up to 6 hours
of use. If you need a white-light torch, then this is a good
choice. However, if you only need a UV torch, then the next one (below)
is neater and brighter.
Its effectiveness (the 390-395nm wavelength) can be seen on a £20. banknote, the "20" is only just visible (i.e. this UV light isn't as good as our best, UV-1 and UV-WHITE, above) - but the other security marking on the note is vivid (see the selection of large photos when you view ref. UV-WHITE).
Go for this one if you want an exceptionally bright wide-area UV light and want to keep the budget under £30.00.
Click on each picture to see it greatly enlarged
A TWENTY POUND NOTE
Here are examples of a British twenty pound note when viewed under various UV lights.
Click on each picture see it greatly enlarged.
is so bright it's causing reflections, but none of the security markings
UV is OK, it does show up the "20", not very brightly, but it
really isn't very bright but you can see the security markings beautifully.
The 100 LEDs produce
the brightest light of all, this is with the light held 20 inches away.
focused light shows up the "20" better than any other UV light.
£39.50, the UV light is the same as the UV-1, above, but it
also has a very bright ordinary (white light) torch
A CREDIT CARD
is an example of a good UV light and a 'bad' UV light when shone on a
British twenty pound note.
Finally, if you want a magnifier with a UV light, be extra careful which one you buy. Below are two sets of photographs showing a ruby, one with an unbranded 10X loupe with UV light (the size happens to be 10X21), one with the QUICKTEST 10X loupe with UV light. These loupes also have a rim of ordinary (white) light, you push the switch one way for white light, the other way for ultra violet light.
For a closer view, click on each of the two images.
Please note that using ultraviolet light is not a 'test' for ruby, because some ruby (especially Burma ruby) fluoresces brilliantly, some ruby does not fluoresce at all; the above is just to show you the difference between the quality of ultraviolet lights (scroll up to read all about wavelengths).
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QUICKTEST, Watford, WD18 8PH, Tel. 01923 220206, email info(at)quicktest.co.uk